Lake clarity measurements have been taken continuously by UC Davis Tahoe Environmental Research Center (TERC) since 1968, when a white disk, called the Secchi disk, could be seen down to 102 feet. It is one of the longest, unbroken clarity records in the world. Secchi depth is the most widely used method of clarity measurement, and the values are consistent with laser-based measurements also taken by TERC researchers. Water transparency in Lake Tahoe is controlled by fine particles blocking light penetration either by scattering or by absorption. The decline in transparency is the result of accumplation of fine sediment particles and growth of small phytoplankton (algae). Fine sediments and nutirents are transporated to the lake by stormwater runoff, stream erosion, and through atmospheric deposition.

By: Data Average Method
Lake Tahoe is designated an Outstanding National Resource Water and a “Waterbody of extraordinary ecological or aesthetic value” by the states of California and Nevada, respectively, for its world famous clarity and striking blue color. Over the past half century however, clarity has significantly diminished. The Lake Tahoe TMDL Program seeks to effectively guide efforts to restore historic clarity within the lake so people may once again be able to see to depths of nearly 100 feet. Annual average Secchi disk depth measurements recorded at the Lake Tahoe Index Station (1968 through 2017). Each annual value is an integrated average using 18 to 37 individual measurements. The trend line (dashed line) is determined statistically using a general additive model (GAM) and used to assess long term trend in clarity. The TMDL Clarity Challenge target is a five year average of 23.8 meters (78 feet) by 2026. The five-year running average (red line) between 2012 and 2017 was 21.3 meters (70 feet).

 Transparency of Lake Tahoe as measured by the annual average Secchi depth.